1. Field of the Invention
The present invention relates generally to medical methods and apparatus. More particularly, the present invention relates to methods and devices for restricting spinal flexion in patients having back pain or other spinal conditions.
A major source of chronic low back pain is discogenic pain, also known as internal disc disruption. Patients suffering from discogenic pain tend to be young, otherwise healthy individuals who present with pain localized to the back. Discogenic pain usually occurs at the lower lumbar discs of the spine (FIG. 1). Pain is typically exacerbated when patients put their lumbar spines into flexion (i.e. by sitting or bending forward) and relieved when they put their lumbar spines into extension (i.e. the standing position, or arching backwards). Discogenic pain can be quite disabling, and for some patients, can dramatically affect their ability to work and otherwise enjoy their lives.
Such discogenic low back pain can be thought of as flexion instability and is related to flexion instability that is manifested in other conditions. The most prevalent of these is spondylolisthesis, a spinal condition in which abnormal segmental translation is exacerbated by segmental flexion.
Current treatment alternatives for patients diagnosed with chronic discogenic pain are quite limited. Many patients follow a conservative treatment path, such as physical therapy, massage, anti-inflammatory and analgesic medications, muscle relaxants, and epidural steroid injections, but typically continue to suffer with a significant degree of pain. Other patients elect to undergo spinal fusion surgery, which commonly requires discectomy (removal of the disk) together with fusion of adjacent vertebrae. Fusion is not usually recommended for discogenic pain because it is irreversible, costly, associated with high morbidity, and of questionable effectiveness. Despite its drawbacks, however, spinal fusion for discogenic pain remains common due to the lack of viable alternatives.
An alternative method, that is not commonly used in practice, but has been approved for use by the FDA, is the application of bone cerclage devices that can encircle the spinous processes or other vertebral elements and thereby create a restraint to motion. Physicians typically apply a tension or elongation to the devices that applies a constant and high force on the anatomy, thereby fixing the segment in one position and allowing effectively no motion. The lack of motion allowed after the application of such a device is thought useful to improve the likelihood of fusion performed concomitantly; if the fusion does not take, these devices will fail through breakage of the device or of the spinous process to which the device is attached. These devices are designed for static applications and are not designed to allow for a dynamic elastic resistance to flexion across a range of motion. The purpose of bone cerclage devices and the other techniques described above is to almost completely restrict measurable motion of the vertebral segment of interest. This loss of motion at a given segment gives rise to abnormal loading and motion at adjacent segments, leading eventually to adjacent segment morbidity.
Recently, a less invasive and potentially more effective treatment for discogenic pain has been proposed. A spinal implant has been designed which inhibits spinal flexion while allowing substantially unrestricted spinal extension. The implant is placed over one or more adjacent pairs of spinous processes and provides an elastic restraint to the spreading apart of the spinous processes which occurs during flexion. Such devices and methods for their use are described in U.S. Patent Publication No. 2005/02161017A1, published on Sep. 29, 2005, and having common inventors with the present application.
As illustrated in FIG. 2, an implant 10 as described in the '017 application typically comprises an upper strap component 12 and a lower strap component 14 joined by a pair of compliance members 16. The upper strap 12 is shown disposed over the top of the spinous process SP4 of L4 while the lower strap 14 is shown extending across the bottom of the spinous process SP5 of L5. The compliance member 16 will typically include an internal element, such as elastomeric members 72a and 72b (FIG. 7 of the '017 application) which are attached to inelastic cables 76a and 76b in such a way that the cables may be “elastically” or “compliantly” pulled apart as the spinous processes SP4 and SP5 move apart during flexion. In particular, the compliance or elasticity is provided by the cables compressing the elastomeric members 72a and 72b between stoppers elements 78a, 78b, 80a, and 80b at their respective ends. In this way, the implant provides an elastic tension on the spinous processes which provides a force that resists flexion. The force increases as the processes move further apart and the rubber or elastomeric blocks become more compressed. Usually, the straps or cables themselves will be essentially non-compliant so that the degree of elasticity or compliance may be controlled and provided solely by the nature of the elastomeric members in compliance members 16.
While potentially robust over millions of cycles of use, the “compressive” compliance members of the '017 application can have difficulty in providing controlled elastic tension within the relatively low 25 N/mm to 75 N/mm range set forth in the application. The use of compressive rubber or elastomeric blocks in the compliance members also limits the length of device elongation which can be achieved. Even if the initial compression provided by the block is within the target elastic resistance range, the stiffness of the compressive block would be expected to rise quickly and potentially fall outside of the target range as the block is further compressed by pulling of the spinous processes on the upper and lower straps. Moreover, even such relatively “low” stiffnesses above 25 N/mm can present some risk of damage or trauma to the spinous processes and other parts of the vertebrae and spine. In order to reduce the compressive force and increase the compressive length, the size of the compressive block may be increased. Increasing the size of the compressive block, however, increases the overall size of the device and is undesirable. The need to have the straps or cables traverse the entire length of the compressive block also increases the size and complexity of the implant structure. Increasing the size of the device is undesirable for many reasons, including making implantation more difficult, while increasing the complexity of the device is undesirable as it increases the risk of failure.
For these reasons, it would be desirable to provide improved spinal implants and methods for their use in inhibiting flexion in patients suffering from discogenic pain. It would be particularly desirable if the improved devices could reliably and repeatedly provide relatively low initial tension on the spinous processes and a relatively low elastic resistance to flexion, even over relatively long lengths of travel. Moreover, any risk of damage to the vertebrae of spine should be minimized. In addition, the devices should have a relatively small size with a decreased complexity in order to facilitate implantation and reduce the risk of failure. Furthermore, the devices should be designed to continue to function even after being cycled for long periods of time (e.g. up to multiple years of implantation) through high numbers of cycles (e.g. up to millions of cycles) and as such should exhibit primarily elastic behavior with minimal plasticity, i.e., low creep. At least some of these objectives will be met by the inventions described hereinbelow.
2. Description of the Background Art
US Patent Publication No. 2005/0216017A1 has been described above. US 2005/0192581 describes an orthopedic tether which can have a stiffness from at least 1 N/mm to at least 200 N/mm and which can be used for many purposes, including wrapping spinous processes. Other patents and published applications of interest include: U.S. Pat. Nos. 3,648,691; 4,643,178; 4,743,260; 4,966,600; 5,011,494; 5,092,866; 5,116,340; 5,180,393; 5,282,863; 5,395,374; 5,415,658; 5,415,661; 5,449,361; 5,456,722; 5,462,542; 5,496,318; 5,540,698; 5,562,737; 5,609,634; 5,628,756; 5,645,599; 5,725,582; 5,902,305; Re. 36,221; 5,928,232; 5,935,133; 5,964,769; 5,989,256; 6,053,921; 6,248,106; 6,312,431; 6,364,883; 6,378,289; 6,391,030; 6,468,309; 6,436,099; 6,451,019; 6,582,433; 6,605,091; 6,626,944; 6,629,975; 6,652,527; 6,652,585; 6,656,185; 6,669,729; 6,682,533; 6,689,140; 6,712,819; 6,689,168; 6,695,852; 6,716,245; 6,761,720; 6,835,205; 7,029,475; 7,163,558; Published U.S. Patent Application Nos. US 2002/0151978; US 2004/0024458; US 2004/0106995; US 2004/0116927; US 2004/0117017; US 2004/0127989; US 2004/0172132; US 2004/0243239; US 2005/0033435; US 2005/0049708; US 2006/0069447; US 2006/0136060; US 2006/0240533; US 2007/0213829; US 2007/0233096; Published PCT Application Nos. WO 01/28442 A1; WO 02/03882 A2; WO 02/051326 A1; WO 02/071960 A1; WO 03/045262 A1; WO 2004/052246 A1; WO 2004/073532 A1; and Published Foreign Application Nos. EP 0322334 A1; and FR 2 681 525 A1. The mechanical properties of flexible constraints applied to spinal segments are described in Papp et al. (1997) Spine 22:151-155; Dickman et al. (1997) Spine 22:596-604; and Garner et al. (2002) Eur. Spine J. 5186-5191; Al Baz et al. (1995) Spine 20, No. 11, 1241-1244; Heller, (1997) Arch. Orthopedic and Trauma Surgery, 117, No. 1-2:96-99; Leahy et al. (2000) Proc. Inst. Mech. Eng. Part H: J. Eng. Med. 214, No. 5: 489-495; Minns et al., (1997) Spine 22 No. 16:1819-1825; Miyasaka et al. (2000) Spine 25, No. 6: 732-737; Shepherd et al. (2000) Spine 25, No. 3: 319-323; Shepherd (2001) Medical Eng. Phys. 23, No. 2: 135-141; and Voydeville et al (1992) Orthop Traumatol 2:259-264.